It is known in the prior art that the control of non-linear combustion process operations such as combustion chamber pressure in boilers, furnaces or soaking pits is influenced by automatic control of the exhaust stack damper position. A piston-type servomotor utilizing compressed air is linked to the damper to control its movement. Many exhaust stacks rely on natural draft to draw the flue gas upward for discharge into the atmosphere. It is the reliance on natural draft only which introduces marked non-linearities in the relationship between the combustion chamber pressure and stack damper position. Contributing to this non-linearity is the resistance to flow introduced by the presence of a recuperator or economizer through which the spent gases pass, for purposes of preheating the air of combustion in the case of a furnace, or for preheating the water in the case of a boiler. Also, the stack draft itself tends to fall as the flow of flue gases increases, because the draft is a function of the flow as well as of the temperature difference between the top and the bottom of the stack. The updraft or suction at the bottom of the stack tends to be reduced for a constant temperature difference as the velocity of the flue gas coming into the bottom of the stack is increased.
The difficulty of controlling combustion chamber pressure in these environments is compounded by the common practice of discharging the flue gases of a plurality of combustion processes through a common stack. This difficulty is because of the tendency toward interaction of the dampers associated with each independent combustion process. An example of these problems is well illustrated by the circumstances of soaking pit operation.
Soaking pits are located in the steel production cycle between the basic oxygen furnace (BOF) where ingots are produced, and the slabbing mill. The BOF produces ingots of steel, either stainless or carbon steel. These ingots sit outside on trucks for a long period and cool off before they are placed in the slabbing mill. The purpose of the soaking pits is to raise the ingots to the desired rolling temperature, and because it takes quite a while for the heat to penetrate through the large tonnage of metal in each ingot, a soaking pit cycle may take eight hours between charging the ingots in the pit and withdrawing them for passage to the slabbing mill.
A typical arrangement would have these pits in batteries of three, with nine such batteries altogether, for a total of twenty-seven pits. The outlet duct from each of the pits in a battery is connected to a common stack provided for the battery of three pits and through which the flue gases are exhausted to the atmosphere. However, each pit has its own damper, which is a hinged, butterfly-type valve, moved by a piston-type servo motor that operates with compressed air. There is always some minimum opening of the pit damper to afford purging of the pit.
In industrial combustion processes, natural draft is commonly relied upon to draw the exhaust gases into the atmosphere, rather than induced draft fans, so that movement of the damper on one pit changes the pressure in the common duct and creates an interaction within the battery of pits as each damper moves in an attempt to maintain the pressure in its respective pit. There can be a constant instability that persists between the three pit pressure control systems, even through the combustion control, which is on the front-end of each pit for controlling the air and gas ratios, works well. Because the differential pressure is higher across their valves, interaction in the control of air and gas is not as significant as contrasted with interactions which take place through the stack.
The exhaust stack arrangement is such that an attempt is made to recover some of the heat going up the stack and transmit it to the input air of combustion through the use of recuperators which preheat the air before it gets to the burners.
The gases from each pit in a given battery flow through a separate exhaust system with its own damper, and a common stack is provided after the individual dampers. The natural draft characteristic of the system is critical in that it changes the normal pressure drop versus flow relationships and the damper position controller gain relationship to flow. Control of the damper position on each pit is very sensitive because the flow rates are not that high and changes in damper position cause changes in draft within a matter of two or three seconds.
The combustion controls associated with each pit respond to the independent variable of temperature which is the product of the combustion process. Temperature in the pit regulates the gas flow, and the gas flow regulates the air flow, and as is commonly found on combustion systems, a cross-coupling and ratio adjustment exists between the air and gas flows arranged so that neither can get out of step with the other while providing a desired fuel quantity and air/fuel ratio.
The control for each damper senses the actual static pressure in the associated pit, and this pressure should be controlled quite closely. If it is too low, or a highly negative pressure, then cold air can be sucked in and the pit cannot properly heat up the ingots. Ingot scaling will also occur. If it is too high, or a positive pressure, the hostile atmosphere within the pit might be blown out into the shop which is both a fire hazard and dangerous to shop personnel.
Conventional pit firing utilizes a modulated firing technique in which the temperature excursions in a temperature versus time profile continuously modulate the fuel gas flow to maintain whatever temperature is specified in the profile for that pit. The fuel gas flow would be fully on when the pit is first started, and gradually tapers off to some very low value as the desired soaking temperature is approached. These pits are not only capable of being fired in the conventional way with modulated firing, but can also be fired with a technique called pulse-firing, with which the pit interactions discussed earlier can become very troublesome.
Pulse-firing creates tremendous external disturbances which are not present with modulated firing where there is some trimming taking place all the time. This is because the pulse-firing technique involves firing the pit at full blast to bring it up to desired temperature as quickly as possible. The fuel gas is then cut off completely and the pit cools down slightly, from about 2480.degree. F. to 2372.degree. F. At 2372.degree. F. the fuel gas is turned on again. About 21/2-3 minutes later, when the temperature again reaches about 2480.degree. F., the gas is turned off again. The firing operation keeps pulsing up and down like this all the time, with the pulse being simply a complete full blast on or nothing at all. As the soaking condition is approached, the duration of the off time increases because the whole atmosphere surrounding the ingots is hotter and more uniform in temperature, so there is less decay in temperature when the gas is turned off. An indication that soaking temperature has been reached is provided by the increase in duration of the off time to a certain magnitude.
The nature of the pulse-firing technique is such that it introduces very severe changes in the input fuel gas flow every time the pulse occurs. Thus, the interaction between the pits can become very significant. Clearly, both the magnitude and rate of change in flow are large when this occurs and the effect on the other pits is severe. Further, because the pits are at different stages in their heating cycles, there can be no synchronism in the sequence of pulsing in the respective pits.
The pulse-firing technique has many advantages over modulated firing. Two of the major benefits of the pulse firing technique are improved heat utilization and increased yield of ingots through reduced scale formation. These stem from the fact that the pulse-firing technique exhibits only two states; firing the pit at full bore or not at all. With modulated firing, a very low fuel gas flow requires a lot of excess air to provide the necessary turbulence, which is not very efficient. When firing at full bore under pulse firing, on the other hand, the air/fuel ratio can be adjusted downward so that it is much closer to stoichiometric and nearer to a slightly reducing atmosphere. This also makes the excess air much lower and therefore the ingots do not scale as much leading to a better ingot yield. Overall, pulse-firing is much more efficient in terms of heat utilization.
It can be readily seen that the pulse-firing technique greatly magnifies the aforementioned interactions between the individual pit damper control systems and greatly complicates the operation of any one damper control system. This is because the typical 2,400 pounds per hour of fuel gases which were flowing suddenly stop. The sudden curtailment of fuel gas flow in any particular pit calls for a step change in damper position owing to the sudden increase in natural draft as the affected damper shuts down, and the other pits close in their dampers in response to the sudden increase in draft. There is a tendency for a damper movement on one pit to affect the control of pressure in the others, leading to instability unless the damper position controllers are detuned.
The present invention overcomes these problems, resulting in a control system which exhibits greater stability and higher speed of improved accuracy and response over a 100% load range. This invention has commercial value because of the large number of pits and furnaces to which part or all of the invention applies whether they are operating in batteries or alone. The improved accuracy and response of the system permits the adjustment of the furnace pressure set point closer to atmospheric pressure, to reduce the amount of cold air leaking in through the furnace cover, with an associated saving in fuel gas being provided.